As a technique to compress a dynamic range, a picture is separated into an illumination component and a reflection component (Hereafter, it is called “a contrast component”), and the illumination component is compressed. The dynamic range with high contrast may be compressed.
As to video contents of an input picture, the illumination component of the picture generally corresponds to an outward light or an illumination light, and becomes a uniform brightness component for an object. In other words, the illumination component is a low spatial frequency component. Accordingly, a smoothing filter subjected to the input picture is an effective means to extract the illumination component of the picture. For example, a low-pass filter is often used as the smoothing filter.
As a filter for separating the illumination component, in general, a Gaussian filter (linear filter) to operate convolution with fixed filter coefficient may be used.
In case of using the linear filter for separating the illumination component, a brightness blur (Hereafter, it is called “a halo”) often occurs at a boundary of an object in the picture. The reason of occurrence is as follows. In case of using the linear filter, an illumination component picture including the boundary of the object is uniformly subjected to the filter processing. As a result, an error signal is mixed at a boundary between adjacent objects.
As a method for reducing the halo, by using a plurality of linear smoothing filters each having a different scale, a result obtained by each smoothing filter is added with weighted-ratio. However, improvement effect by this method is insufficient. In order to reduce the halo, the illumination component must be extracted with high spatial accuracy.
Another method for reducing the halo is disclosed in JP-A 2003-8935 (Kokai). In this patent reference, an epsilon filter (non-linear filter) having a superior ability to preserve edges is used as the filter for separating the illumination component. Furthermore, by controlling a threshold for the epsilon filter, a boundary of the object is suitably extracted. As a result, occurrence of the halo is reduced.
However, in this patent reference, the boundary of the object (an edge element including a segment or a character) is suitably extracted from the illumination component. Accordingly, in case of compressing the illumination component, the boundary of the object is also compressed. Furthermore, the boundary of the object is extracted not from the contrast component but from the illumination component. Even if the contrast component is composed with the illumination component, the boundary of the object is not included in the contrast component. Briefly, by compressing the illumination component, the boundary of the object is also compressed.